The long term objective of this proposal is to determine the mechanism(s) responsible for the changes in airway reactivity to bronchoconstrictor and bronchodilator agents which occurs during maturation. Our hypothesis is that these maturational changes are closely related to increased levels of plasma hydrocortisone and altered circulating catecholamines concentrations. During maturation, plasma hydrocortisone and catecholamines will determined by radioimmuno and radioenzymatic assays, respectively. It is proposed to chronically catheterize the guinea pig femoral vein to allow repeated samplings of plasma during maturation. The hydrocortisone and catecholamine concentrations in the blood will be temporally related to measurements of airway reactivity to histamine in vivo. Airway reactivity will be determined using whole body plethysmography by measuring changes of specific airway conductance. Further, it is hypothesized that the elevated hydrocortisone concentrations affect membrane receptors (histaminergic, cholinergic, adrenergic, 5-HT, leukotriene and ouabain) by changing their affinities, densities and/or coupling so that, subsequent to receptor activation, intracellular concentrations of the second messengers, cyclic AMP, cyclic GMP and inositol and activation of ATPase are modified. Consequently, biological response will be changed. Membrane receptor properties will be evaluated from dose response curves to agonists before and after irreversible antagonism and in airway muscle membrane fractions using radioligand binding assays. In addition, the membrane will be characterized by determining membrane potential and the contribution of the electrogenic sodium pump using standard electrophysiological techniques. The ability to activate adenylate cyclase and cyclic nucleotides will be determined in guinea pig and bovine tracheal smooth muscles before and subsequent to agonist stimulation in vitro using radioimmunoassay techniques while phosphatidylinositol formation will be determined using labelled substrate and subsequent chromatographic separation of the metabolic products for scintillation counting. All of these measurements will be made in tissues from immature and mature animals as well as in tissues treated either in vivo or in vitro with hydrocortisone. An understanding of these regulatory processes will provide a better knowledge of the events and interactions which determine muscle contractility in health and disease. The proposal may specifically provide further insights Into mechanisms of aging and, in particular, on the mechanisms responsible for the remission of bronchial asthma in children at puberty.